Portable electronic devices are becoming smaller and smaller, which accordingly causes the volume and capacity of their batteries to decrease. In order to improve endurance of the batteries, power supply modules of these electronic devices are required to provide a stable output voltage when the battery voltage varies in a wide range. Buck-boost converters thus are widely used in these applications.
FIG. 1 illustrates a traditional buck-boost converter with four transistors. Energy is stored in the inductor L when the transistors S1, S3 are on and the transistors S2, S4 are off. The stored energy is then provided to a load when the transistors S1, S3 are off and the transistors S2, S4 are on. Since the transistors S1˜S4 keep switching during operation, power loss of this traditional buck-boost converter is large.
To reduce the power loss, different working modes, such as BUCK mode and BOOST mode, are adopted. In the BOOST mode, the transistor S1 is maintained on and the transistor S2 is maintained off, while the transistors S3 and S4 are controlled using a fixed frequency peak current control scheme. In the BUCK mode, the transistor S4 is maintained on and the transistor S3 is maintained off. The transistors S1 and S2 are controlled by the fixed frequency peak current control scheme.
However, logic used to determine the working mode in prior arts is complicated. And abrupt mode transition could induce spikes on the output voltage.